Scientists are seeking permission to carry out experiments that would result in children being born with three biological parents. UK medical authorities say they will almost certainly approve the application in the next few weeks.

The aim of the technique is to prevent mothers passing on degenerative genetic diseases to their children. But campaigners say it could lead to significant increases in elderly women having children. They also claim it represents an unacceptable step towards the creation of designer babies.

'By creating a child with three genetic parents, these scientists are taking the first step towards genetic engineering of human beings. That is not a direction in which we should be going,' said Dr David King, director of Human Genetics Alert.

The technology - which is being developed by a team at Newcastle University - will involve the implanting of the nucleus of an embryo from an affected mother into an egg taken from a donor that has been stripped of its nucleus.

Human eggs carry small spherical or rod-like bodies called mitochondria, which supply energy for the growing foetus. These have their own genes, inherited separately from the child's main chromosomal genes.

Crucially, some mitochondrial DNA is defective and can pass on one of around 50 degenerative diseases. It is thought more than 1,000 children in Britain suffer from diseases caused by defective mitochondria, some ending up with chronic brain disease.

Neurologist Professor Doug Turnbull and embryologist Dr Mary Herbert believe that by implanting the nucleus of an embryo of a mother with defective mitochondria into the egg of a woman with healthy mitochondria, the resultant foetus will be free of the destructive genetic diseases.

The Newcastle team, which has applied for permission from the Human Fertilisation and Embryology Authority (HFEA) to carry out the experiments, stress they are at least three years away from offering the treatment. If they get approval they will experiment on abnormally fertilised eggs that have come from IVF treatment which would otherwise have been discarded.

'At the moment there is no cure for these children who are often very ill with a poor quality of life,' said Turnbull. 'We are at a very early stage of this work, but it does offer the potential to help many mothers and their families.'

However, the prospect horrifies groups who believe the ethical problems and safety risks far outweigh any benefits to future generations. Dr Jamie Grifo, of New York University, applied to carry out similar experiments but was prevented from doing so in America. He then carried out operations in China using a technique similar to that planned by Turnbull and Herbert, removing the nuclei from one set of eggs and implanting them into another set of emptied eggs with healthy mitochondria. A Chinese woman became pregnant as a result, but lost both infants after six months.

Dr King said: 'These new techniques of reproductive technology, which involve more and more manipulation of embryos, are potentially very risky for the child.'

Patrick Cusworth, of the charity Life, condemned the work, saying it would raise questions as to who would be the real mother.

'We would also be concerned about the safety of any embryo or child born from this method and worried that the technology could be abused by other scientists for different purposes.'

The claims were dismissed by Paul Preston, of The Children's Mitochondrial Disease Network. He has three children who suffer from inherited diseases.

'This research offers us real hope and could be a vital step forward,' he said. 'Anybody who has to live with a child that is unaware of the environment he lives in, who cannot move and can only eat or drink through a tube does not realise the suffering caused by such diseases. This is not about genetic engineering but about stopping children suffering.'

A spokeswoman for the HFEA confirmed it had received an application for the research but would not comment on when or if it was likely to be approved.